The Transformational Power of Frequency Phase Transfer Methods for ngEHT
Round 1
Reviewer 1 Report
This manuscript briefly describes frequency phase transfer methods and their impact on ngEHT science. As a theorist, I found this manuscript a useful introduction to these methods. In a few areas, there are opportunities to improve the clarity for other non-specialists.
1. In the introduction, brief equations or even proportionalities describing the relevant physical effects and their evolution with frequency would be useful to better understand why FPT methods are possible.
2. Approximately equal frequencies are sometimes used interchangeably through the text (e.g., 85 vs. 86 GHz, 230 vs. 240 GHz). It would be good to standardize the reference frequency quoted, when possible.
3. Can the authors elaborate on the statement “Furthermore [FPT methods] are potentially applicable for space VLBI”? Naively, I would think FPT would be completely irrelevant for space observations.
4. In some figures, it appears that some labels are over-plotted on top of each other in larger font, perhaps for clarity when preparing talks. The authors should remove the extra labels.
5. I had trouble understanding section 4.4, the impact of non-integer frequency ratios. What are phase ambiguities? Is there a a way to arrive at +/-120 degrees from a frequency ratio of 340/255, for example? Most likely, only a few sentences of clarification are necessary.
6. MultiView should be defined.
Author Response
----Reviewer 1----
This manuscript briefly describes frequency phase transfer methods and their impact on ngEHT science. As a theorist, I found this manuscript a useful introduction to these methods. In a few areas, there are opportunities to improve the clarity for other non-specialists.
1. In the introduction, brief equations or even proportionalities describing the relevant physical effects and their evolution with frequency would be useful to better understand why FPT methods are possible.
This is a reasonable suggestion. On the other hand copious details are provided in our recent review; we have attempted here to provide a more digestible version. Nevertheless we have added a sentence to address this comment.
2. Approximately equal frequencies are sometimes used interchangeably through the text (e.g., 85 vs. 86 GHz, 230 vs. 240 GHz). It would be good to standardize the reference frequency quoted, when possible.
It is our intention to use the integer ratios where applicable; the actual bands are many decades of GHz wides. We have corrected a few places where we misused the label.
For example, where not applicable would be for the `nominal' ngEHT frequencies are 230 and 340, now plus 86GHz. These are the common names. Furthermore the SgrA* sizes were measured at a reference frequency of 86GHz.
3. Can the authors elaborate on the statement “Furthermore [FPT methods] are potentially applicable for space VLBI”? Naively, I would think FPT would be completely irrelevant for space observations.
FPT was discussed for VSOP-2 and demonstrated (to a limited extent) on RadioAstron. It is planned for Millimetron. We have expanded the text, but this is really another research topic.
4. In some figures, it appears that some labels are over-plotted on top of each other in larger font, perhaps for clarity when preparing talks. The authors should remove the extra labels.
We have fixed a number of the figures
5. I had trouble understanding section 4.4, the impact of non-integer frequency ratios. What are phase ambiguities? Is there a a way to arrive at +/-120 degrees from a frequency ratio of 340/255, for example? Most likely, only a few sentences of clarification are necessary.
We have expanded the text
6. MultiView should be defined.
This sentence provides a succinct definition: MultiView \citep{rioja_17} with simultaneous observations of multiple sources has the potential to provide a precise astrometry. Also, in the next parragraph "The calibrator solutions can be spatially interpolated... results in an effective angular separation equal to zero degrees".
We feel the best place to find more details are the provided references
With our best regards!
Reviewer 2 Report
Dear Authors,
I have carefully studied the paper and enjoyed it while reading. Certainly this work deserves to be published after minor corrections.
Few comments I'm providing below:
(I) In my opinion, it would be worth providing a brief justification of why the 86 GHz band is considered, and not, for example, 43 GHz. As far as I remember, ALMA, for example, is still planning to manufacture Band 1? Is this dicatated by the need to observe the heavily scattered Sgr A* (115 GHz, which fall within the 86 GHz receiver band)?
(II) Page 7, line 262-263: Maybe it would be useful to give a relationship that shows the connection between the ratio of frequencies and the magnitude phase jump that occur after the calibration?
(III) Some sentences in my opinion are overloaded and should be split into several. It seems to me that it could make the material easier to read and comprehend.
Examples of such:
1) Lines 16-19: The very first sentence
2) Lines 28-33: "Next-generation..."
3) Lines 72-76: "The Frequency Phase Transfer..."
4) Lines 152-156: "It comprised..."
5) Lines 234-238: "Figure 4 allows us to...."
(IV) A few typos:
1) Page 5, caption to fig. 1: brackets "Left)" and "Right)" are missing or extra brackets
2) Page 7, line 251: extra comma "being longer, ."
3) Page 9, line 293: "will be AN integer ratio"
4) Page 12, line 386: typo "(\nu_{low} and \nu_{low})"
(V) The text refers to 85 GHz, but in some places 86 GHz is also mentioned. Probably it is better to bring it to a single value so that won't confuse the reader.
(VI) Please, check the placement of commas.
With my best regards,
Reviewer.
Author Response
----Reviewer 2----
I have carefully studied the paper and enjoyed it while reading. Certainly this work deserves to be published after minor corrections.
Few comments I'm providing below:
(I) In my opinion, it would be worth providing a brief justification of why the 86 GHz band is considered, and not, for example, 43 GHz. As far as I remember, ALMA, for example, is still planning to manufacture Band 1? Is this dictated by the need to observe the heavily scattered Sgr A* (115 GHz, which fall within the 86 GHz receiver band)?
The referee is correct. We have discussed the various frequency options internally at great length. We found that the 43GHz is not the best solution in general. We have added text concerning one of the reasons, the larger frequency ratio, to the text in Sec. 4.3. The other issues are more relevant to the other papers in this special issue. One of the other more compelling arguments is that the ngEHT needs to differentiate itself from the ngVLA, for example. This is not something that we would comment on in a paper about frequency phase transfer.
(II) Page 7, line 262-263: Maybe it would be useful to give a relationship that shows the connection between the ratio of frequencies and the magnitude phase jump that occur after the calibration?
Added
(III) Some sentences in my opinion are overloaded and should be split into several. It seems to me that it could make the material easier to read and comprehend.
Examples of such:
1) Lines 16-19: The very first sentence
Fixed
2) Lines 28-33: "Next-generation..."
Fixed
3) Lines 72-76: "The Frequency Phase Transfer..."
It is long, but we think breaking it only separates closely linked concepts.
4) Lines 152-156: "It comprised..."
Fixed
5) Lines 234-238: "Figure 4 allows us to...."
It is long, but we think breaking it only separates closely linked concepts. We have added ")" symbols to the list items.
(IV) A few typos:
1) Page 5, caption to fig. 1: brackets "Left)" and "Right)" are missing or extra brackets
Fixed perhaps. Is this not a common form to address the left and right hand side of the figure?
2) Page 7, line 251: extra comma "being longer, ."
Fixed
3) Page 9, line 293: "will be AN integer ratio"
Fixed
4) Page 12, line 386: typo "(\nu_{low} and \nu_{low})"
Fixed
(V) The text refers to 85 GHz, but in some places 86 GHz is also mentioned. Probably it is better to bring it to a single value so that won't confuse the reader.
It is our intention to use the integer ratios where applicable; the actual bands are many decades of GHz wides. We have corrected a few places where we misused the label.
For example, where not applicable would be for the `nominal' ngEHT frequencies are 230 and 340, now plus 86GHz. These are the common names. Furthermore the SgrA* sizes were measured at a reference frequency of 86GHz.
(VI) Please, check the placement of commas.
Apologies. We have reviewed the paper for these errors
With our best regards!
